Polyurethanes have been known and used for many years, and the basic general chemistry for their preparation, the reaction of a polyol, a polyisocyanate and a chain extender, is well documented.
A polyol frequently used for this purpose is PTMEG. In some applications, especially where a polyurethane of high modulus and hardness is needed, it is desirable to use a PTMEG with a molecular weight of over about 2000. Unfortunately, a PTMEG of such high molecular weight is difficult to make and use because its high viscosity hinders handling and subsequent reaction.
It has now been found that this difficulty can be eliminated by coupling, with formaldehyde, two, three or four segments of PTMEG, each having a number average molecular weight of 1000-3000. This gives an oligomeric formal diol with a number average molecular weight of 2000-12,000, and with a viscosity much lower than that of a PTMEG of equivalent molecular weight. Moreover, when such a formal diol is used to prepare a polyurethane, the product shows no significant degradation of properties compared to one made with an equivalent PTMEG.